The annual domestic consumptions of chicken and chicken eggs are reported to be as high as 2,200,000 tons and 2,500,000 tons, respectively. The worldwide consumptions of chicken and chicken eggs are about 83,000,000 tons and about 56,000,000 tons, respectively. On the other hand, inedible feathers and eggshell membranes are discharged in large amounts. Protein makes up 90% or more of feathers and eggshell membranes. The main component of feathers is keratin protein, and that of eggshell membranes is collagen-like protein. Both of them contain much cysteine (about 10%), and their decomposition products are known to have many physiological functions such as antioxidative action.
An eggshell membrane covers an egg cell (egg yolk) and an egg white in a chicken egg, and physically isolates them from the outside world together with the egg shell, thereby protecting the chicken egg from harmful ultraviolet rays, oxygen, and drying. In addition, eggshell membranes have an important function of protecting egg yolk and white from infection by adventives such as germs and viruses. In addition, eggshell membranes contain antibacterial substances and antibacterial enzymes such as lysozyme and β-N-acetylglucosaminidase. Eggshell membranes are also reported to contain other special proteins, and thus its functionality is receiving attention.
Various functional products containing eggshell membranes are known. Examples of the known products include a hair permanent agent containing eggshell membranes for protecting hair cuticle (Japanese Unexamined Patent Application Publication No. 2000-128744) and a court plaster containing an eggshell membrane decomposition product for wound healing (Japanese Unexamined Patent Application Publication No. 2003-225298).
Most parts of eggshell membranes are disposed of without use. The reason for this is that eggshell membranes are refractory and thus difficult to handle, and that there is no effective method for solubilizing eggshell membranes. At present, eggshell membranes are mainly treated by an acid or alkali, but this technique presents the problems of reddish browning color and development of malodor caused by amino acid decomposition. In addition, the yield of useful components is low because of excess reaction. As an alternative technique, protease treatment is studied (for example, see Patent Documents 1 to 4), but this technique is associated with the problems such as that (1) pretreatment processes such as pulverization, ultrasonication, and boiling are necessary, (2) long treatment is necessary, and (3) the decomposition rate is low (about 20%). The enzyme reaction proceeds under relatively mild conditions, so that the increase of the yield of active components is expected, but there is still no practical technique because of the above-described problems.